Re: [PATCH 2/2] Rewrite jump_label.c to use binary search

[Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>]

* Jason Baron (jbaron@redhat.com) wrote:
> On Wed, Sep 22, 2010 at 11:02:50AM -0400, Mathieu Desnoyers wrote:
> > * Andi Kleen (andi@firstfloor.org) wrote:
> > > 
> > > >
> > > >>> +	for (; entry < stop && entry->key == key; entry++)
> > > >>> +		if (kernel_text_address(entry->code))
> > > >>
> > > >> This does not work for modules I'm afraid, only for the core kernel. You
> > > >> should test for __module_text_address() somewhere.
> > > >
> > > > I thought it was shared now, but ok.
> > > 
> > > Double checked. This is ok because kernel_text_address()
> > > already checks for modules. You were probably thinking
> > > of __kernel_text_address()
> > 
> > Ah right,
> > 
> > Although we have another problem:
> > 
> > __module_text_address() includes module init text, which defeats the
> > purpose of the check put in there by Jason.
> > 
> > So the check works for the core kernel, but not for modules.
> > 
> > Mathieu
> > 
> 
> it works for modules too...it does:
> 
> struct module *__module_text_address(unsigned long addr)
> {
>         struct module *mod = __module_address(addr);
>         if (mod) {
>                 /* Make sure it's within the text section. */
>                 if (!within(addr, mod->module_init, mod->init_text_size)
>                     && !within(addr, mod->module_core,
> mod->core_text_size))
>                         mod = NULL;
>         }
>         return mod;
> }
> 
> and then in kernel/module.c we have :
> 
> 
>         module_free(mod, mod->module_init);
>         mod->module_init = NULL;
> 
> 
> So, I was relying on the fact module_init gets set to NULL after the
> free happens. However, there a small race there in that the vfree()
> happens before module_init() is set to NULL. So that is probably most
> easily fixed be wrapping those two lines with the jump_label_mutex.

It's both module_init = NULL _and_ init_text_size = 0 that make sure
the test "within(addr, mod->module_init, mod->init_text_size)" is valid.
Just the "module_init = NULL" can cause problems with addresses in the
low range of kernel addresses. With a long enough module init section,
the offset from NULL can end up (temporarily) in the kernel address
range.

But this is all wrong: __module_text_address is relying on
preempt_disable() to ensure coherency of this test is just racy, as you
point out above. So we either do the RCU synchronization properly, or
hold the module_mutex around the module text address test _and_ actual
access to the module init section.

Thanks,

Mathieu

> 
> thanks,
> 
> -Jason
> 
> 

-- 
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com